iser-target: move rdma wr processing to a shared function

isert_put_datain() and isert_get_dataout() share a lot of code
in rdma wr processing, move this common code to a shared function.

Use isert_unmap_cmd to cleanup for RDMA_READ completion.
Remove duplicate field in isert_cmd and isert_rdma_wr structs
Change misc debug messages to track isert_cmd

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Vu Pham <vu@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>